Acknowledgements

We thank Inger Merete S. Paulsen, Ida M. Jalk, Zhila Nikrozi, Helle Høyer, Mette F. Vistisen and Lotte V. Holbech for expert technical assistance. We also thank King-Wai Yau for helpful discussions. Funding for JP and SN comes from the Water and Salt Research Center at the University of Aarhus, established and supported by the Danish National Research Foundation (Danmarks Grundforskningsfond). DG, TT, and PA were funded by the National Institutes of Health.